Blocky inversion of gravity and magnetic data

Blocky 3D linear inversion of gravity and magnetic data. Include the planting algorithm and traditional smooth depth-weighted inversion in the same class. Using a moving footprint to optimize Jacobian matrix building.

Checklist:
- [ ] Create/update docstrings
- [ ] Make tests for new code
- [ ] All tests pass
- [ ] Can be merged
- [ ] Changelog entry
- [ ] Benckmarks